The present invention relates in general to modulation systems for frequency shift keying with an upstream Gaussian filter, referred to as Gaussian minimum shift keying (GMSK) modulation systems. In particular, it relates to an improved GMSK filter for such GMSK modulation systems.
GMSK modulation is frequently used in present-day cordless telephone systems or mobile radio systems. In these modulation systems, with an upstream Gaussian filter, referred to as GMSK modulation systems, a carrier signal is modulated with a Gaussian-filtered digital data signal. Frequency modulation (FM) or quadrature modulation may be used for modulation in this case. Since quadrature modulation requires linear I and Q paths which match one another very exactly, and also requires a phase shifter and a mixing module, it is relatively complex to implement. Thus, for cost reasons, frequency modulation is frequently used, since it is simpler to implement.
Frequency modulation uses a voltage controlled oscillator, referred to as a VCO. The digital data signal used for modulation is for this purpose filtered by a Gaussian filter. The Gaussian filter ensures that the digital square-wave signals, which represent the actual data signal, are smooth to a certain extent. To a certain extent, this represents a low-pass filter and ensures that no excessively abrupt sudden phase changes occur. It is thus possible to produce a relatively narrowband modulated carrier signal. The signal produced at the output of the Gaussian filter then drives the voltage controlled oscillator (VCO).
The Gaussian filter can be implemented in various ways. For example, it may be in the form of an analog filter element with discrete components, as is used in the Siemens cordless DECT telephones. Alternatively, it may be in the form of a digital filter, as is used, for example, in the Phillips and NSC cordless telephones.
In the GMSK filters normally used until now, digital preprocessing followed by digital/analog conversion was carried out by an X-bit digital/analog converter (D/A converter). As for any digital/analog conversion, the digital/analog conversion necessarily results in quantization errors due to the step function used in the D/A converter. The quantization errors can be reduced by reducing the size of the steps used in the D/A converter, by increasing its resolution and hence its bit length.
Conventional D/A converters, which operate on the current source principle, have binary-weighted current sources. The individual current sources in this case emit currents which represent a binary multiple of a reference current Iref. They thus have magnitudes Iref, 2*Iref, 4*Iref . . . 2{circumflex over ( )}N*Iref. Any digital value can thus be produced by simple addition. One problem with these D/A converters is that, when switching to the most significant bit (MSB), switching is carried out from the sum of all the reference currents, apart from the largest reference current, to the largest reference current. If the reference currents are now not precisely matched, which is virtually always the situation in practice, there is a sudden change in the converter characteristic. This can result in the production of radio-frequency sideband signals, which contravene specified sideband suppression.
In D/A converters having a voltage output, reference voltages are added. The voltages can be added actively via a buffer, or passively via resistors. In the passive version, however, the output resistance is not constant, and has a relatively high value. Furthermore, resistors are not particularly suitable for integration, since they require a large surface area for their implementation. The voltages are therefore normally added via a buffer that requires a sufficiently wide bandwidth (in this case in the order of magnitude of 10 MHz). However, such a buffer also requires a relatively large surface area for its implementation, and consumes a large amount of current.
In addition, conventional D/A converters require a digital filter. This is frequently in the form of a table stored in a read only memory (ROM).
The GMSK filters that are used are intended to be implemented on a minimum surface area in the course of the ever greater miniaturization of electronic appliances. At the same time, however, the filters are intended to be as accurate as possible.
International Patent Disclosure WO 97/04525 describes a digital GMSK filter which adds currents from a large number of individual current sources to produce a total current which is converted to a corresponding voltage value by a resistor. The current sources, which are weighted in accordance with the desired filter function, are in this case driven via a shift register.
International Patent Disclosure WO 97/33414 A specifies a configuration for pulse-shaping for GMSK modulation. The digital data stream received by the configuration is converted by read only memories, logic circuits and a digital/analog converter to an analog control signal for a VCO.
A further digital-to-analog converter for producing a GMSK-weighted analog signal is described in Published, European Patent Application EP 0 743 759 A1.
It is accordingly an object of the invention to provide a digital GMSK filter which overcomes the above-mentioned disadvantages of the prior art devices of this general type, whose accuracy is as high as possible and which can be implemented on a minimal surface area.
With the foregoing and other objects in view there is provided, in accordance with the invention, a digital Gaussian minimum shift keying (GMSK) filter for frequency modulating a carrier signal in a GMSK transmission system. The GMSK filter contains a control logic module having a shift register receiving modulation bits. The shift register has a first side for receiving logic 1 values of the modulation bits and a second side receiving logic 0 values of the modulation bits. The second side is opposite the first side, and the shift register has a shift direction switchable between a stop, a left and a right shift direction. A number of individual current sources are provided and output individual current values with the individual current sources being individually driven by the control logic module in accordance with a digital signal to be modulated. An output resistor is coupled to the individual current sources with a total current from the individual current sources being converted by the output resistor to a voltage value for controlling a voltage controlled oscillator, by which a frequency of the carrier signal is modulated.
The object is achieved by the digital GMSK filter according to the invention. The filter uses a parallel D/A converter with a current output for the D/A converter. The analog output signal in this case consists of a total current, which is obtained by additive combination of individual currents from individual current sources.
The current sources that are used for the filter according to the invention are what are referred to as differential current sources. The expression xe2x80x9cdifferential current sourcexe2x80x9d is in this case intended to mean that they each supply the current which is necessary to move from one step on the converter characteristic to the next step. The necessary chip surface area to provide the filter is in this case governed only by the maximum total current, and not by the number of current sources.
The individual currents from the differential current sources can be converted directly in an external resistor to the control voltage required for driving the VCO, so that, in contrast to the D/A converters known from the prior art, no output buffer is required. The current values from the differential current sources are in this case not weighted linearly, but have Gaussian weighting. Therefore, there is no need for any digital filtering.
The configuration of the shift register according to the invention (input capability on both sides and capability to shift the switch direction) results in that the individual current sources can be driven in an advantageous manner.
Particularly when varactors are used for this purpose, voltage controlled oscillators (VCOs) have a wide scatter in their operating parameters. The drive voltage must therefore be trimmed during production, in order to achieve a predetermined frequency shift in the VCO. The modulation shift can be carried out, in the GMSK filter according to the invention, by trimming the reference current of the differential current sources.
Overall, the digital GMSK filter according to the invention provides a filter that can be implemented on a small chip surface area, which is governed only by the maximum total current. The filter does not require an output buffer. The Gaussian weighting of the currents from the differential current sources results in that there is no need for any digital filtering. Furthermore, this allows an exact Gaussian converter characteristic to be achieved.
In accordance with an added feature of the invention, the individual current values of the individual current sources are weighted such that the total current has a Gaussian characteristic, as a result of a respective connection and disconnection of in each case one of the individual current sources.
In accordance with another feature of the invention, the shift register is operable in a long loop mode, in which the shift register is always filled either entirely with xe2x80x9clogic onesxe2x80x9d or entirely with xe2x80x9clogic zerosxe2x80x9d, and the shift register is operable in a short loop mode, in which the shift direction for specific positions in the shift register is changed, and a shift process is suppressed for one cycle.
In accordance with a further feature of the invention, for the long loop mode in the shift register, the shift direction is changed as a function of a next bit to be modulated.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a digital GMSK filter, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.